Regulatory air pollution limits for diesel engines have caused some manufacturers to adopt selective catalytic reduction (SCR) technology for reducing nitrogen oxides (NOx) in engine exhaust. The SCR process introduces a diesel exhaust fluid (DEF), such as an NOx reducing compound (e.g., a urea water solution), into the hot exhaust gas. The DEF chemically reduces pollutant compounds, such as NOx, into non-pollutant compounds in conjunction with a catalyst.
Introduction of DEF into exhaust is typically achieved by a doser (e.g., a port) injecting (e.g., spraying) an aqueous DEF solution as a stream of small droplets into a stream of exhaust. However, during typical operation of an engine, the conditions for mixing dosed DEF with exhaust and the conditions for the SCR process are not optimal. For example, the temperature of the exhaust is not optimal (i.e., either too hot or too cold), the amount of DEF introduced to the exhaust is not optimal (i.e., either too much or too little DEF is introduced into the exhaust), or DEF does not sufficiently mix with the exhaust. When DEF introduced into exhaust that does not undergo the SCR process (e.g., too much DEF for the exhaust temperature), DEF crystals will accumulate within the exhaust system, both on the interior surface of the exhaust pipe and at the DEF doser. Buildup of DEF crystals in the exhaust system detrimentally affects the performance of the exhaust system, and also is indicative of inefficiency in the SCR process: DEF crystals represent both wasted DEF solution and reduced SCR efficiency. During injection of DEF into the diesel exhaust stream during SCR there is a significant amount of the total DEF injected that contacts the walls of the exhaust pipe and becomes a liquid wall film. While this process occurs, the DEF that is wetting the pipe walls does not reach the catalyst for its intended use and the intended quantity of reactant is not available in the catalyst.
Recent developments in engine aftertreatment system design have been moving increasingly towards more compact, smaller volume systems. From the viewpoint of DEF dosing and decomposition, this is diametrically opposite to what is most desirable. In order to compensate for the negative impact caused by these trends, a very large emphasis has been place upon mixer design and to facilitate more efficient DEF decomposition to produce NH3, both of which has met with limited success. A system is needed to reduce the negative effects from DEF injection and mixing with exhaust to reduce the amount of DEF that wets the pipe walls and reduce the amount of crystallization of DEF that does not undergo the SCR process, while still providing efficient mixing of DEF with exhaust for the SCR process.